1. Field of the Invention
The invention in general relates to the growing of large silicon carbide crystals and more particularly to silicon carbide boules of high purity and high crystal quality for use in semiconductor devices.
2. Description of Related Art
Silicon carbide is being used extensively as a semiconductor material for various electronic applications. Semiconductor devices of silicon carbide have the ability to operate at higher voltages and temperatures than conventional silicon or gallium arsenide devices thus providing for higher power devices with reduced cooling requirements. Electronic applications for silicon carbide semiconductor devices include compact, low cost airborne, ship and ground-based solid state radars, including transmitters and power supplies, aircraft engine and flight controls, electric tank and ship propulsion systems, temperature tolerant power conditioners for aircraft and military vehicles and satellite communications, to name a few. In the commercial sector, silicon carbide may be used in the field of high definition television, industrial power conditioning, personal communication systems, nuclear control and instrumentation and electric vehicle power trains.
Silicon carbide crystals may be grown in a variety of ways, one of which utilizes a furnace system into which is introduced gaseous sources of silicon and carbon. The silicon source is constituted by silane gas, the use of which requires an extensive investment in safety equipment since silane is highly toxic and explosive.
In another process known as physical vapor transport, a seed crystal of silicon carbide is positioned within a furnace system which also includes a source, or feedstock, in the form of silicon carbide powder. The feedstock is heated to a particular temperature, with the seed crystal maintained at a different temperature whereby the silicon carbide sublimes and silicon carbide is deposited upon the seed crystal forming a boule. After the boule is grown to the desired size, it is removed from the furnace system and then prepared and sliced into wafers for use as semiconductor device substrates.
During the growth process, the feedstock becomes depleted and the apparatus may be partially disassembled in order to replenish the silicon carbide powder. This time consuming procedure is repeated until the desired silicon carbide crystal boule length is achieved. The present invention obviates this time consuming operation.
In addition, silicon carbide powder feedstock is not available in an ultra high purity, semiconductor quality grade. Consequently, the grown single crystal silicon carbide typically includes unwanted contaminants, leading to degraded operation. The present invention additionally provides for ultra high purity, semiconductor grade silicon carbide single crystal boule or epitaxial layer growth.